Application to geophysics: Challenges and some solutions

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Presentation on theme: "Application to geophysics: Challenges and some solutions"— Presentation transcript:

1 Application to geophysics: Challenges and some solutionsAndrew Binley

2 Hydrogeophysics – the driversCharacterising groundwater systems is challenging because of the (physical and chemical) complexity of the shallow subsurface and the difficulty in observing the structure of the system …Hartman et al. (2007)… and the complex response due to external loading.Robin Nimmer, Moscow, Idaho

3 Hydrogeophysics – the driversGeophysics has been widely used to support groundwater investigations for many years. However, many of the earlier approaches concentrated on using geophysics to define lithological boundaries and other subsurface structures.Resistivity profile and hydrogeological section, Penitencia, CA (after Zohdy, 1964).

4 Hydrogeophysics – the driversDuring the 1990s there was a rapid growth in the use of geophysics to provide quantitative information about hydrological properties and processes.Much of this was driven by:- the recognition of the importance of heterogeneity of subsurface properties that influence mass transport in groundwater systems.- the need to gain information of direct value to hydrological models, particularly given the developments of ‘data hungry’ stochastic hydrology tools.Tiedeman & Hsieh (2004)

12 Larger scale exampleElectrical resistivity tomography (ERT) provides an assessment of vertical structureC+C-P+P-C+C-P+P-C+C-P+P-C+C-P+P-C+C-P+P-C+C-P+P-C+C-P+P-C+C-P+P-Current is injected between C+ and C-The voltage difference between P+ and P- is measuredThe voltage difference is a function of the currentinjected and the resistivity beneath the electrode array

14 Challenge 2: Data fusionResistivity & Induced PolarisationGPRBoreholelogsGround ConductivityLocal sampling and geologyHow do we bring all these data together to formone consistent, improved model of the system?

15 Challenge 2: Data fusionCan we use other information to help constrain the inversion of geophysical data?For example, we may be able to estimate spatial covariancestructure based on well log data?Linde, Binley, Tryggvason, Pedersen and Revil (2006)

16 Challenge 2: Data fusionWe could jointly invert the two (or more) data using a structural similarity, e.g. by minimising the cross-gradients operatorIn areas where the gradients are in the same or opposite direction (or where one of the gradients is zero) t will be zero (and the pixels structurally similar)Gallardo (2006)

18 Challenge 3: Assessing information contentAt times there is a need to assess information content in data (this has been significantly overlooked to date)Understanding the value of different information will permit appropriate resource allocation to the project and help with survey design.This is becoming more and more relevant as large hydrological projects invest in hydrogeophysical surveys.£Xdrilling£Xgeophysics

25 e.g. permeability structureCoupled hydrogeophysical inversionSurely we know something about the hydrology?And, if so, then we should use this in our inversionHydrological model?Geophysical surveys(assumedknown)Rock physicsmodel(s)Hydrological model,e.g. permeability structureInversion

26 Coupled hydrogeophysical inversionDo we need to invert geophysical data?We have been exploring the potential of using geophysical data (not images) as a means of constraining hydrological models in an McMC framework.Scholer, Irving, Binley and Holliger (2011)

27 Coupled hydrogeophysical inversionPrior distribution for the 4 hydrological model parametersPosterior distribution for the 4 hydrological model parameters for each of the 4 layersScholer, Irving, Binley and Holliger (2011)

28 SummaryDeterministic inversion of 3D geophysical data is now relatively common, although the assessment of uncertainty is lacking.Attempts have been made to jointly invert geophysical data, although most of these have been done in 2D.We need to develop ways of combining multiple data (multiple scales).These fusion approaches must allow some assessment of information value, particularly as we look at new survey designs (for future data).Attempts have been made to use geophysical data within a hydrological model inversion. So far these have been limited to relatively low dimensional models.